Steel cold work tool, its use and manufacturing

ABSTRACT

The invention concerns a steel article, which consists of an alloy having a chemical composition, which contains in weight-%: 1.2 to 2.5 C; 0.8 to 2.0 Si, which partly can be replaced by aluminium, which may exist in an amount of max 1.0%; 0.1 to 1.5 Mn; 0.5 to 1.5 Cr; 1.2 to 5.0 (V+Nb/2), however max 1.0 Nb; balance iron and impurities in normal amounts, and having a microstructure which contains 4 to 12 volume-% of MC-carbides. The steel article can be used for manufacturing of cold-work tools, particularly pilger rolls for cold rolling of tubes. The invention also relates to a method of manufacturing the article.

TECHNICAL FIELD

The invention relates to a steel article, its use for the manufacturingof cold-work tools and a method for the manufacturing of the article.

BACKGROUND OF THE INVENTION

High demands are raised on materials for cold-work steels, particularlyfor certain applications, which demands cannot be satisfiedsatisfactorily with those materials which for the time being arecommercially available. This is particular true in connection withapplications where abrasive wear is a dominating problem, at the sametime as the object shall have an adequate toughness. An application ofthat kind is for rolls for cold rolling of stainless steel andparticularly rolls for the manufacturing of stainless tubes, anapplication for which the invention has specifically been developed. Theinvention, however, can be applied also for other types of cold-worksteels, as for example for tools for cold-extrusion, powder-pressing,and deep drawing.

A steel grade which today is used for rolls for pilger rolling ofstainless tubes is known under its trade name SR1855 and has the nominalcomposition 0.96 C, 1.50 Si, 0.80 Mn, 1.0 Cr balance iron and impuritiesin normal amounts. That steel provides an adequate toughness to productslike pilger rolls for the manufacturing of stainless tubes. The steel,which is manufactured in a conventional way, however, has anunsatisfying wear resistance and also bad surfaces because of largecarbides of M₃C-type. An other steel which has been tested for pilgerrolls is the steel grade which is powder metallurgical manufactured andwhich is known under the registered trade mark VANADIS®4 and which hasthe nominal composition 1.5 C, 1.0 Si, 0.4 Mn, 8.0 Cr, 1.5 Mn, 4.0 V,balance iron and impurities in normal amounts. Also the standardisedhot-work steel SS2242 has been used for pilger rolls. That steel has thenominal composition 0.39 C, 1.0 Si, 0.4 Mo, 5.2 Cr, 1.4 Mn, 0.9 V,balance iron and impurities in normal amounts. Further, the standardisedhigh-speed steel M1 is used and also the commercial, powdermetallurgical manufactured high-speed steel VANADIS®23, which has thenominal composition 1.28 C, 4.4 CR, 5.0 Mo, 6.4 W, 3.1 V, and normalamounts of Mn, Si and unavoidable impurities. The last mentioned steelhas a very good wear resistance but does not provide an adequatetoughness to the product. Besides, these steels are comparativelyexpensive because of their high content of alloying elements, and/orbecause of the powder metallurgical manufacturing.

BRIEF DISCLOSURE OF THE INVENTION

The object of the invention is to provide a material for cold-work rollsfor pilger rolling of stainless tubes, but which also can be used forother cold-work applications, and which combine a very good wearresistance, particularly a very good resistance against abrasive wear,with an adequate toughness of the product. This can be achieved throughthe chemical composition of the steel which is a characteristic featureof the invention, in combination with a manufacturing technique, whichneither is conventional (in order to avoid undesirably large carbidesbecause of the slow cooling process in connection with conventionalingot production and/or continuous casting), or powder metallurgical,which provides too small carbides for the achievement of the desiredwear resistance of the product.

The chemical composition of the steel of the invention is stated in theappending patent claims and will be commented more in detail in thefollowing.

The structure of the steel of the article according to the invention,after hardening and in tempering substantially consists of temperedmartensite, which contains 4 to 12 volume-% of carbides of MC-type,which are precipitated during the solidification process, at least about80 volume-%, preferably at least 90 volume-%, of the carbides having asize which is larger than 1 μm but smaller than 20 μm.

In order to achieve the above mentioned carbide dispersion sometechniques can be employed which are known per se. In the first placethe so called spray forming technique is recommended, which is alsoknown under the name the OSPREY-method, in connection with which acasting successively is established therein that a melt in the form ofdrops are sprayed against the growing end of the ingot which iscontinuously manufactured due to the fact that the drops solidifycomparatively rapidly once they have hit the substrate, however not asfast as in connection with powder manufacturing and not as slow as inconnection with conventional manufacturing of ingots or in connectionwith continuous casting. By employing this technique, the precipitated,above mentioned, MC-carbides more particularly will obtain sizessubstantially within the lower part of the said size range of 1 to 20μm, more specifically within the range 1 to 10 μm, and typically withinthe range 2 to 10 μm.

Another useful technique is ESR-remelting (Electro Slag Remelting),particularly for the manufacturing of products with larger dimensions,i.e. with diameters from Ø350 mm and up to 600 mm. By employing thistechnique the precipitated, above mentioned, MC-carbides moreparticularly will obtain sizes substantially within the upper part ofthe said size range of 1 to 20 μm, more specifically within the range 3to 20 μm, and typically within the range 5 to 20 μm.

As far as the various alloying elements in the steel are concerned, thefollowing applies.

Carbon shall exist in a sufficient amount in the steel in order, on onehand, together with vanadium and possibly existing niobium to form 4 to12 volume-% of MC-carbides, where M substantially is vanadium, and onthe other hand to exist in solid solution in the matrix of the steel inan amount of 0.8 to 1.1%, preferably 0.9 to 1.0%. Suitably, the contentof carbon that is dissolved in the matrix of the steel is about 0.95%.The total amount of carbon in the steel, i.e. carbon that is dissolvedin the matrix of the steel plus that carbon that is bound in carbides,shall be at least 1.2%, preferably at least 1.3%, while the maximumcontent of carbon may amount to 2.5%, preferably max. 2.3%.

According to a first preferred embodiment of the invention, the steelcontains 1.7 to 2.0 carbon, preferably 1.75 to 1.9 carbon, nominallyabout 1.8 carbon, in combination with nominally about 3.6 vanadium inorder to provide a total content of MC-carbides amounting to 6 to 12,preferably 7 to 10 volume-% of MC-carbides, in which vanadium partly canbe replaced by the double amount of niobium.

According to a second preferred embodiment, the steel contains 1.5 to1.8, preferably 1.55 to 1.7, and suitably nominally about 1.6 carbon, incombination with nominally about 2.3 vanadium, which partly possibly canbe replaced by the double amount of niobium in order to provide 4 to 8,preferably 4 to 6 volume-% of MC-carbides in the steel.

Silicon, which partly can be replaced by aluminium, shall, together withpossibly existing aluminium, exist in a total amount of 0.8 to 2%,preferably in an amount of 1.2 to 1.8%, most suitably in an amount of1.3 to 1.7% or as a nominal content of about 1.5% in order to increasethe carbon activity in the steel and hence contribute to the achievementof an adequate hardness of the steel without creating brittlenessproblems because of dissolution hardening at too high contents ofsilicon. The aluminium content however, must not exceed 1.0%.Preferably, the steel does not contain more than max 0.1% Al.

Manganese and chromium shall exist in the steel in a sufficient amountin order to give the steel an adequate hardenability. Manganese also hasthe function to bind those residual amounts of sulphur, which can existin low contents in the steel by forming manganese sulphide. Manganesetherefore shall exist in an amount of 0.1 to 1.5%, preferably in anamount of at least 0.2%. A most suitable content lies in the range 0.4to 1.2%, most conveniently in the range 0.7 to 1.1%. The nominal contentof manganese is about 0.8%.

Chromium shall exist in the steel in order, together with manganese, togive the steel a hardenability, which is adapted to its intended use.Hardenability in this connection means the ability of the hardening topenetrate more or less deep in the object that is hardened. Thehardenability shall be sufficient for the object to be hardened down toa certain depth from the surface, so that in the surface region ahardness is achieved after hardening and tempering which amounts to 58to 62 HRC, while in the centre of the object, or at a depth of 30 mmfrom the surface or deeper, there is obtained a hardness which does notexceed 40 HRC after hardening and tempering. For the achievement ofthis, the chromium content shall amount to 0.5 to 1.5%, preferably 0.7to 1.3% and most suitably to 0.9 to 1.15%. The nominal chromiumcomposition is about 1.0%.

Vanadium shall exist in the steel in an amount of at least 1.2% and max5.0%. Preferably, the content of vanadium shall lie in the range 1.8 to4.2% in order to form MC-carbides together with carbon. In principle,vanadium can be replaced by niobium. But for this twice as much niobiumis needed as compared with vanadium, which is a drawback. Besides,niobium will cause the carbides to adopt a more edged shape and theywill also be larger than pure vanadium carbides, which can initiatefractures or chippings and consequently reduce the toughness of thematerial. Therefore niobium must not exist in an amount of more than max1.0%, preferably max 0.5%. Most advantageously the steel should notcontain any intentionally added niobium, which in the most preferredembodiment of the steel therefore should not be tolerated more as animpurity in the form of residual elements from the raw materials usedfor the manufacturing of the steel.

According to said first preferred embodiment, the content of MC-carbidesin the material shall amount to 6 to 12 volume-%. The content ofvanadium in this case should amount to at least 3.2% and to max 4.2%,preferably be 3.4 to 4.0%, suitably max 3.8%. The nominal content ofvanadium according to this first embodiment is 3.6% vanadium.

According to the above-mentioned second, preferably chosen embodiment,the content of vanadium should be at least 1.8% and max 3.0%, andsuitably lie in the range 1.9 to 2.5%. The nominal content of vanadiumin this case is about 2.3%.

The steel need not, and should not, contain any more alloying elementsin significant amounts in addition to the above-mentioned alloyingelements. Some elements are definitely undesired, because they have anundesired influence on the features of the steel. This e.g. is a casefor molybdenum and tungsten, which form undesired carbides. Molybdenumalso strongly increases the hardenability of the steel, which is againstone of the purposes of the invention, namely to provide a tough core inthe product. Molybdenum and tungsten therefore preferably should notexist as intentionally added elements, which can be tolerated in anamount of max 0.3 and max 0.6, respectively, but should preferably notexist more than as unavoidable impurities in an amount of max 0.05% ofeach of them.

Phosphorous should be kept as low as possible in order not to impair thetoughness of the steel. Also sulphurous is an undesired element, but itsnegative impact on the toughness can substantially be neutralised bymeans of manganese, which forms essentially harmless manganesesulphides. Sulphur therefore can be tolerated in a maximum amount of0.05%, preferably max 0.02%. Nickel is another undesired element becauseof its hardenability effect and should therefore not exist in an amounthigher than 0.3%, preferably not more as an unavoidable impurity. Thetotal amount of nickel, molybdenum, and copper should not exceed 0.5%,preferably not exceed 0.25%. Nitrogen exists as an unavoidable impurityin the steel but does not exist as an intentionally added element.

Cobalt can be tolerated in an amount of max 1.0% as an indifferentelement. Cobalt, however, is an expensive element and should thereforenot exist more than as an unavoidable impurity from the used rawmaterials.

In the manufacturing of the steel article according to the invention,first a melt a prepared in a conventional way by melting necessary rawmaterials, adjusting the alloy, desoxidation, and desulphurisation. Theningots can be made from this melt by employing some conceivabletechniques, depending on the desired carbide sizes in the finished,hardened and tempered steel, which in its turn depends on the intendeduse of the steel. If comparatively small carbides are desired, whichmeans that at least 80 volume-% shall have sizes within the range 1 to10 μm, preferably within the range 2 to 10 μm, suitably the sprayforming technique is employed, which technique is also known by itstrade name OSPREY. More information about this technique can be found inan article having the title: “The production of advanced materials bymeans of the OSPREY process” by A. G. Leatham et al in ModernDevelopments in Powder Metallurgy, Vol. 18-21, 1988, issued by MetalPowder Industries Federation, Princeton, N.J.

If instead somewhat coarser carbides are desired, which means that atleast 80 volume-% shall have sizes within the range 3 to 20 μm,preferably 5 to 20 μm, a number of ingots can be cast from the melt,with sizes suitable for electrodes for ESR-remelting (Electro SlagRemelting), the ingots thereafter being ESR-remelted in order to formingots for further processing. The produced ingots, whether they areproduced by spray forming or by ESR-remelting, then are forged and/orrolled to desired dimensions for the achievement of the articleaccording to the invention

At the manufacturing in laboratory scale, which shall be described inthe following, however, none of the above mentioned techniques have beenemployed. Nor has the whole process sequence been applied for thepreparation of the metal melt, which briefly is described above andwhich is employed for full-scale production. Instead 50 kg laboratoryheats were manufactured by melting measured quantities of alloyingelements in order, as close as is possible by means of that simpletechnique, to obtain nominal compositions of the experimental materials.Thereafter the melt was cast in uninsulated moulds, in which the meltwas allowed to cool, so that ingots were obtained having octagonal, 150mm cross-section. The ingots then were forged to size Ø60 mm. Microscopestudies of the thus obtained materials, which had a chemical compositionof the invention, showed that the desired size distribution ofMC-carbides of the invention, see above, was achieved. This indicatesthat the manufacturing technique, which provides ingots having the saiddimension, makes it possible to precipitate MC-carbides having thedesired size and quantity during the solidification process, whilelarger undesired carbides are not formed. This also can be said to be ameasure of the solidification rate, which is desirable for theachievement of the carbide structure of the invention. However, thisdoes not mean that the ingots according to the invention shall bemanufactured in these dimensions at a commercial production. At thecommercial production of ingots with larger dimensions, such asaccording to the OSPRAY technique and/or according to the ESR-technique,the cooling is intensified, at least this being true as far as theOSPRAY-technique is concerned, because of the nature of the technique,so that the end result, as far as the carbide sizes are concerned, canbe that which is achieved at said laboratory manufacturing of smalleringots.

Further features and aspects of the invention will be apparent from thepatent claims and from the following, detailed description of theinvention and from performed experiments.

BRIEF DESCRIPTION OF DRAWINGS

In the following, the invention will be explained more in detail andperformed experiments shall be described, reference being made to theaccompanying drawings, in which

FIG. 1 shows the principle design of a pilger roll for cold rolling ofstainless tubes;

FIG. 2 shows the pilger roll in cross-section along the line II—II inFIG. 1;

FIG. 3. shows the microstructure of an experimental material;

FIG. 4 shows the impact strength and the hardness of examined materials;and

FIG. 5 is a bar chart showing the wear of some examined experimentalmaterials.

DETAILED DESCRIPTION AND PERFORMED EXPERIMNTS

When cold rolling tubes, such as tubes of stainless steel, according tothe pilger rolling process, two opposite rolling mill rolls are used, inthis text denominated rolls 1, of the type which is shown in FIG. 1 andFIG. 2. The two rolls have a tapered groove 2 covering approximatelyhalf the circumference of the rolls. The groove starts with a dimension,which is equal to that of the hot rolled tube, which is the startingmaterial for the pilger rolling, and tapers towards the final size. Acentral boring for a not shown shaft is designated 4.

During the rolling, the rolls are subjected to a rapid movement fore-andbackwards. The rolling is performed during the forward movement. Atpilger rolling very big reductions are possible, up to 90%. Forstainless steel tubes 50 to 70% are common values. Thus, one pass atpilger rolling is equivalent to 3 to 5 passes at cold drawing. Thevelocity is between 40 to 100 strokes/min and the tube feeding isbetween 4 and 15 mm/stroke. It should be understood that the stresses onthe pilger rolls, which are used at the above-described cold-workingoperation, are very high. Therefore, particularly the wear resistance inthe groove 2, which is the active working part for the forming of a tubehas to be very good, at the same time as the toughness in a surfacelayer 5 has to be sufficient in order to prevent chipping, and thetoughness of the entire tool has to be adequate in order to preventtotal failure because of brittle fracture. Thus the centre portion 3 ofthe tool, which has been indicated by dotted lines in FIG. 2, betweenthe groove 2 and the centre hole 4, should have a very good toughness.

The centre part 3 the tool material thus shall have a low hardness,which gives sufficient toughness to the whole tool 1, while the roll 1in the region 5 of the groove 2 down to a certain depth measured fromthe surface shall have a hardness of 58 to 62 HRC and a very high wearresistance, and a sufficient toughness in the core of the article inorder to prevent complete failure of the article and in the surfaceregion to prevent chippings. The same principle is applicable also forother types of cold-work tools than pilger rolls. The said hardeningdepth, however, may vary depending on the intended use of the steel fordifferent types of tools, and the dimensions and shapes of the tools.For certain applications, a hardening depth of at least about 10 μmmeasured from the surface may be desirable and suitable, while in othercases it is sufficient and/or desirable that the tool has a hardness of58 to 62 HRC only down to a depth of about 3 μm measured from thesurface.

Experiments Based on Production at a Laboratory Scale

A first series of experiments based on production at a laboratory scaleaimed at investigating if a material of the invention can satisfy thesaid requirements of the material in said region 5 in a conceived pilgerroll.

In Table 1, the compositions of steels Nos. 1 to 3 correspond to thenominal composition of the experimental alloys in this first series ofexperiments. Steels Nos. 4 to 6 are experimental alloys, the valuesstated in Table 1 being the analysed compositions of these steels. Thevalues of steels Nos. 7 and 8 are the nominal compositions of a coupleof steels according to the invention having preferably chosencompositions, based on the result from the first series of experiments.Besides the elements mentioned in Table 1, the steels also containedminor amounts of other impurities than those which are stated. Thus theoxygen content of the steels Nos. 4 to 6 amounted to 48, 43, and 41 ppm,respectively. In the table, steels No. 1 and No. 4 are referencematerial of type SR1855.

TABLE 1 Chemical composition, weight-% Steel No. C Si Mn P S Cr Ni Mo VN Balance 1 0.96 1.50 0.80 Max Max 1.0 Max Max Max Max Fe 0.025 0.0200.10 0.07 0.03 0.03 2 1.50 1.50 0.80 Max Max 1.0 Max Max 2.0 Max ″ 0.0250.020 0.10 0.07 0.03 3 2.00 1.50 0.80 Max Max 1.0 Max Max 4.0 Max ″0.025 0.020 0.10 0.07 0.03 4 0.95 1.28 0.84 0.007 0.005 1.23 0.14 0.030.09  0.011 ″ 5 1.43 1.28 0.88 0.008 0.006 1.21 0.15 0.01 1.86  0.016 ″6 1.91 1.17 0.98 0.011 0.008 1.23 0.16 0.01 4.07  0.030 ″ 7 1.8 1.500.80 Max Max 1.0 Max Max 3.6 Max ″ 0.025 0.020 0.10 0.07 0.03 8 1.6 1.500.80 Max Max 1.0 Max Max 2.3 Max ″ 0.025 0.020 0.10 0.07 0.03

50 kg heats were made of the experimental alloys, which were cast inmoulds to form ingots, which were forged to Ø60 mm.

The following material tests were performed:

-   -   Hardness (HB) after soft annealing.    -   Microstructure in soft annealed condition and after heat        treatment 870° C./30 min/oil+300° C./2×2 h, in the surface and        centre of Ø60 mm.    -   Hardness after tempering 300° C./2×2 h for T_(A)=870°        C./min/oil.    -   Wear testing against SiO₂-paper. T_(A)=870° C./min/oil+300°        C./2×2 h.    -   Impact testing with unnotched test specimens at 20° C., LT2.        T_(A)=870° C./min/oil+300° C./2×2 h.        Hardness After Soft Annealing

At the working of cold-work tools, such as for example pilger rolls, bymeans of cutting tools, it is desirable that the hardness in the softannealed condition is not too high. The soft annealed hardness of thesteels 5 and 6 were measured to 249 HB and 269 BB, respectively, whichis satisfactory. The reference material, steel No. 4, had a softannealed hardness of 241 HB.

Microstructure

The microstructure in the soft annealed condition and after heattreatment at 870° C./30 min/oil+300° C./2×2 h at the surface and in thecentre of the rod which had the size Ø60 mm was examined. The amount ofMC-carbides with sizes within the size range which is characteristic ofthe invention, see above and the appending patent claims, increased withthe increased vanadium content, and it was stated that also the vanadiumcarbides were evenly dispersed in the material. In FIG. 3 themicrostructure in the soft annealed condition of steel No. 6 is shown.

Hardness After Hardening and Tempering

According to the listed requirements of the invention it is desirablethat the surface hardness of the finished tool is 58 to 62 HRC, mostpreferably at least 60 HRC. In FIG. 4 the hardness of the test materialsafter austenitising at T_(A)=870° C./30 min/oil, quenching in oil andtempering at 300° C./2×2 h is shown.

Toughness

The results of tensile testing performed at room temperature withunnotched test specimens is also shown in FIG. 4 for steels Nos. 4, 5and 6. The toughness is reduced with increased vanadium content but isjudged still to be sufficient to prevent chipping in the surface layerof the tool.

Abrasive Wear

The resistance to abrasive wear is a critical material feature ofparticularly pilger rolls but also of cold-work tools for several otherapplications. The wear resistance was examined via pin-to-disk-test withSiO₂ as an abrasive agent. The chart in FIG. 5 shows that the wearresistance of steels No. 5, and particularly steel No. 6 was stronglymuch better than of the reference material steel No. 4. The testmaterials had been hardened from 870° C./30 min, quenched in oil andtempered at 300° C./2×2 h.

The material tests, which were performed with specimens made of thethree laboratory heats, showed that a high content of MC-carbides, whereM substantially is vanadium, is necessary for the achievement of adesired abrasive, but also adhesive wear resistance. Particularly steelNo. 6 satisfies that requirement. That steel also satisfies therequirement as far as desired surface hardness is concerned.

Experiments Based on Full Scale Production

By the employment of conventional steel manufacturing technique therewere manufactured full scale beats of steels having the chemicalcompositions according to Table 2.

TABLE 2 Chemical composition, weight-% Steel No. C Si Mn P S Cr Ni Mo VN Balance 9 1.51 1.48 0.85 0.029 0.026 0.96 0.1 0.21 2.23 0.049 Fe 101.63 1.26 0.83 0.016 0.0007 1.02 — 0.05 2.38 0.011 Fe

Besides the alloying elements and the impurities stated in Table 2, thesteels only contained iron and other impurities than those mentioned inthe table in amounts which are normal in conventional steelmanufacturing practice.

Steel No. 9, however, unintentionally contained a higher content ofmolybdenum than what is desirable, but below the level which maximallycan be tolerated within a wide tolerance range.

EXAMPLE 1

From steel No. 9 there was cast an ingot with size Ø500 mm by the sprayforming technique; briefly in the following way. Droplets were formed bygas atomisation of a stream of molten steel. The melt droplets wereinitially sprayed against a rotating disc, on which they quicklysolidified by rapid cooling; cooling rate about 10² to 10³° C./s. Aningot was successively established on the plate, size Ø500 mm, and thespraying of droplets was continued towards the growing ingot in a modewhich is known per se until the ingot had achieved the desired length.The obtained ingot was then allowed to cool freely in air, was thenheated to about 100 to 1200° C., and was forged to the shape of barshaving a final dimension of Ø220 mm

Samples were taken from the surface and from the centre of one of themanufactured bars. Soft annealed samples had a hardness of about 260 HB(Brinell hardness). The samples were hardened by heating to 870° C./30min and then quenched in oil, whereafter the samples were tempered at300° C./2+2 h. The hardness, impact strength of unnotched samples at 20°C., wear resistance against SiO₂-paper and microstructure of thehardened and tempered samples were examined. The following values wereachieved:

-   Hardness: about 61 to 62 HRC, mean value 61.5 HRC-   Impact strength (impact energy): 12 J (surface sample)-    13.5 J (centre sample-   Wear resistance (loss of weight) 8.9 mg/min (source sample)-    8.8 mg/min (centre sample)-   Microstructure (carbide sizes): >80 volume-% of the carbides in the    surface samples had a size of 1 to 5 μm, mean value about 2 to 3 μm-    >80 volume-% of the carbides in centre samples had a size of 2 to    10 μm, mean value about 6 μm

EXAMPLE 2

From steel No. 10 there were manufactured electrodes, which were ElectroSlag Remelted to form an ingot with the dimension □ 400 mm. The ingotwas forged to the shape of bars with the dimension Ø220 mm, from whichsamples were taken, which were heat treated and tested in the same wayas in Example 1. The following values were obtained:

Soft Annealed Samples

-   Hardness 221 HB (surface sample)-    234 HB (centre sample)    Hardened and Tempered Samples (Mean Values)-   Hardness about 59 HRC-   Impact strength (impact energy) about 15 j-   Wear resistance (weight loss) about 11.5 mg/min-   Microstructure (carbide size)    -   >80 volume % of the carbides had sizes in the range 5 to 20 μm-    occasional carbides had sizes up to max 80 μm×10 μm What is claimed    is:

1. Steel article consisting of an alloy having a chemical composition which contains in weight-% 1.2 to 2.5 C, 0.8 to 2.0 Si, which partly can be replaced by aluminium, which may exist in an amount of max 1.0%, 0.1 to 1.5 Mn, 0.5 to 1.5 Cr, 1.2 to 5.0 (V+Nb/2), however max 1.0 Nb, max 0.3 Mo, max 0.6 W, balance iron and impurities in normal amounts, and having a microstructure which contains 4 to 12 volume-% of MC-carbides.
 2. Article according to claim 1, wherein at least about 80 volume-% of the MC-carbides has a size between 1 μm and 20 μm in the hardened and tempered condition of the steel.
 3. Article according to claim 2, wherein at least about 80 volume-% of the MC-carbides has a size in the dimension range 1 to 10 μm in the hardened and tempered condition of the steel.
 4. Article according to claim 2, wherein at least about 80 volume-% of the MC-carbides has a size in the dimension range 3 to 20 μm in the hardened and tempered condition of the steel.
 5. Article according to claim 1, wherein the alloy contains at least 1.3 and max 2.3 C.
 6. Article according to claim 5, wherein the alloy contains 1.8 to 4.2 V.
 7. Article according to claim 6, wherein the alloy contains 1.7 to 2.0 C and 3.2 to max 4.2 V, and that the quantity of MC-carbides in the material amounts to 6 to 12 volume-%.
 8. Article according to claim 6, wherein the alloy contains 1.5 to 1.8C and 1.8-max 3.0, V, and that the quantity of MC-carbides in the material amounts to 4 to 8 volume-%.
 9. Article according to claim 1, wherein the alloy contains 1.2 to 1.8 Si, max 0.5 Al.
 10. Article according to claim 1, wherein the alloy contains max 0.5% Nb.
 11. Article according to claim 1, wherein the alloy contains at least 0.2 Mn.
 12. Article according to claim 1, wherein the alloy contains 0.7 to 1.3 Cr.
 13. Cold-work tool, consisting of a tool made of a steel article according to claim 1 and wherein after hardening and tempering, the tool has a hardness of 58 to 62 HRC in a surface layer (5), while the hardness in the core of the tool is max 40 HRC.
 14. Cold-work tool according to claim 13, wherein the hardness in the surface layer is at least about 60 HRC.
 15. A method of manufacturing a steel article, comprising: preparing a metal melt consisting of an alloy having a chemical composition according to claim 1, continuously making an ingot of the melt, the melt successively being supplied to the ingot which is caused to grow successively, cooling the successively supplied melt to solidify with a velocity corresponding to the solidification velocity which is achieved at any of those continuous processes which include spray forming and ESR-remelting, wherein, during the solidification process, vanadium combines with carbon to form MC-carbides of which at least about 80 volume-%, has a size between 1 and 20 μm.
 16. Article according to claim 1, wherein at least about 90 volume-% of the MC-carbides has a size which is larger than 1 μm but smaller than 20 μm in the hardened and tempered condition of the steel.
 17. Article according to claim 2, wherein at least about 90 volume-% of the MC-carbides has a size in the dimension range 1 to 10 μm in the hardened and tempered condition of the steel.
 18. Article according to claim 17, wherein the dimension range is 2 to 10 μm.
 19. Article according to claim 2, wherein at least about 90 volume-% of the MC-carbides has a size in the dimension range 3 to 20 μm, in the hardened and tempered condition of the steel.
 20. Article according to claim 19, wherein the dimension range is 5 to 20 μm.
 21. Article according to claim 6, wherein the alloy contains 1.75 to 1.9 C, and 3.4 to 4.0 V and the quantity of MC-carbides in the material amounts to 7 to 10 volume-%.
 22. Article according to claim 21, wherein the alloy contains max 3.8 V.
 23. Article according to claim 6, wherein the alloy contains 1.55 to 1.7 C, and 1.9 to 2.5 V, and the quantity of MC-carbides in the material amounts to 4 to 6 volume-%.
 24. Article according to claim 1, wherein the alloy contains 1.3 to 1.7 Si, max 0.1 A1.
 25. Article according to claim 1, wherein the alloy contains at least 0.4 to 1.2 Mn.
 26. Article according to claim 25, wherein the alloy contains 0.7 to 1.1 Mn.
 27. Article according to claim 1, wherein the alloy contains 0.9 to 1.15 Cr.
 28. Method according to claim 15, wherein at least 90 volume % has a size between 1 and 20 μm. 